Combining UV Maps for a Single Object
I am assuming both UV maps are associated with the object?  You can create duplicates of all of your materials.  One material, let's call it A, will use UV map A; material B will use UV map B.  They will otherwise be identical.  You can set the UV map for the material using the "UV Map" input node.

You then need to define which vertices/faces are associated with each UV map.  I suggest using vertex groups to do this.  So you'll have vertex group A and group B.

Finally, you need to assign the right materials to the right vertices.  In edit mode, use your vertex group A to select all of the A vertices.  Then go to the material tab, select your material A in the slot list, and click "Assign."  Do the same for vertex group B/material B.

You'll need to repeat this process for each base material used by your object.